This invention relates to an optical element having diffractive grooves, a metal die for forming it, and a cutting tool for the die, and in particular, to an optical element with a reduced loss of light quantity to make the effective light quantity near to 100%, and a metal die and a cutting tool for obtaining an ideal diffractive optical element by taking into consideration factors such as a shape of diffractive grooves, surface roughness, conditions of working the metal die, a tool for working the metal die, resin material.
A diffraction optical element is an optical element such that sawtooth shaped steps are provided on an optical surface of an optical element, diffraction is generated by varying the phase of a light wave passing there, to utilize the function to deflect the optical path. For a bundle of rays refracted by a basic aspherical shape, by further deflecting the optical path by the effect of diffraction, it can exhibit with a single optical surface a diffraction effect equivalent to that with two optical surfaces. On top of it, an optical path is more difficult to be deflected the longer the wavelength is in the case of refraction, but an optical path through diffraction is more deflected the longer the wavelength is; therefore, the wavelength dependency of refraction can be reduced by combining the both.
For an example of application of a diffractive lens, an image-sensing lens for a camera provided in a personal computer, a pickup optical element for an optical disk etc. can be cited. The former can make chromatic aberration smaller efficiently with a small number of lens pieces by using diffraction effect; therefore, it actualizes an image-sensing lens which is thin, of light weight, and convenient for being provided in a personal computer. Further, for an example of the latter, it can be cited an objective lens which is used in correcting the aberration owing to the wavelength fluctuation of a high-output laser diode as a light source which is generated at the time of writing information after it is read out from an optical disk such as a DVD or a CD.
Further, in order that different optical disks such as a DVD and a CD may commonly use a single optical element, an optical element utilizing a diffraction effect is employed to correct satisfactorily aberration for a plurality of light-source wavelengths and to secure satisfactory chromatic aberration characteristics against wavelength fluctuation owing to temperature variation and a mode hop.
However, in the case of the former lens, if scattering is produced on an optical surface or inside a lens, a flare appears in the formed image to reduce the contrast, which deteriorates the image quality sharply. Especially in the case of a diffractive lens, because of the discontinuous optical surface, it is difficult in designing the lens to make diffraction efficiency 100% for whole incident light in the angle of view, and it has a characteristic such that a certain amount of scattering is produced even if it is ideally produced.
Accordingly, in manufacturing a diffractive lens, in order to reduce scattering by the lens to a level practically of no problem, it is more important than a case of a usual lens to generate the shape of its optical surface which is nearest to the designed shape as much as possible. As for the level of scattering of no practical problem, a level not higher than 5% of incident light quantity, or more desirably a level not higher than 3% is required. This is equivalent to the surface reflectivity of an optical surface made of the representative optical glass such as BK7 in the case where it has not been coated with a reflection reducing coating, and it is the criterion in asking for a necessary image quality and a merit of employing a diffractive optical surface that the loss of light quantity by scattering is at least not more than that by reflection in the state of no reflection reducing coating.
Further, in the case of the latter optical element in a pickup system for an optical disk, because the shortening of life and lowering of reliability are generally more remarkable, the higher power a laser diode outputs, it is preferable to use a laser diode at a low output as much as possible; therefore it is necessary to reduce the loss of light quantity such as scattering in the optical path as much as possible in order to secure a sufficient light quantity in writing.
For a permissible range of the above-mentioned light quantity loss, it is usually obtained a value not larger than 10% of the remainder when an incident light quantity is subtracted by the surface reflection component, or more desirably, a value not larger than 5% of it. This value is empirically obtained by synthesizing such factors as the alignment of the optical element, the light quantity dispersion of laser diodes, the sensitivity dispersion of light receiving devices.
As described in the above, in an optical element utilizing diffraction, as compared to a usual optical element having a continuous optical surface, an influence of scattering etc. is easy to be produced remarkably; accordingly, it is important to obtain an ideal diffraction efficiency without loss of light quantity by scattering, and for that purpose, first of all, it should be mentioned that a diffractive optical surface must be produced in such a manner as to have an ideal shape.
However, in the manufacturing of a conventional diffractive optical element, it is not clear what degree of an error would be practically of no problem for the above-mentioned ideal shape, and in the case where a metal die for forming and transferring a diffractive optical surface is cut-worked, also with respect to the shape of the cutting part of a tool, only it is known that the edge should be made sharp, but it is not clear that to what degree the edge should be sharpened, or what kind of a side effect is produced when it is made sharp.
Further, there has been no idea such that the apex angle of a tool required should be made definite by taking into consideration the parallelism of the step section of diffractive grooves to the incident bundle of rays. Further, also it has not been clear a threshold value of surface roughness to reduce scattering sufficiently at the portion of an optical surface other than the step sections of diffractive grooves. Also for the material to make up the transfer optical surface of a metal die, there has not been a concept that a high-machinability material is necessary in order to maintain the sharp edge of a cutting tool by reducing the wear during working. Because of that, the sharpness of the corner of a cutting part of a tool becomes dull only by cut-working, a small number of metal dies. As the result, generation of an ideal shape of diffractive grooves is made impossible, and the following problems have been frequently observed, which are that scattering of an incident bundle of rays is brought about, taking more time for working the optical surface of a metal die than necessary, that scattering becomes larger by an insufficient precision of working, etc.
This invention was performed by considering main causes of the above-mentioned points, and it is an object of the invention to provide a metal die capable of actualizing the manufacturing of a diffractive optical element having a good efficiency, a tool for the die, and an optical element manufactured by the die.
FIG. 1 is a cross-sectional view of an optical element according to this invention;
FIG. 2 is an enlarged cross-sectional view in the direction of the optical axis showing a portion near one of the diffractive grooves of an optical element;
FIG. 3 is a perspective view showing a double or triple spindle super-precision lathe;
FIG. 4 are drawings showing a tool for cutting a metal die; FIG. 4(a) is a perspective view, FIG. 4(b) is the front view of the cutting part, and FIG. 4(c) is the side view of the cutting part;
FIG. 5 is a graph showing the lowering of diffraction efficiency for a single light-source wavelength in a sawtooth-shaped diffractive optical surface of a flat plate, when diffractive grooves are cut-worked by using a tool having the arc-shaped corner of the cutting part in working the metal die;
FIG. 6 is a drawing showing the relation between wavelength used taken for the abscissa, and the diffraction efficiency of an optical element taken for the ordinate;
FIGS. 7(a) to 7 (c) are drawings showing the relation between setting positions of a metal die and a tool at the time of cut-working;
FIG. 8 is a drawing showing the relation between a tool 70 and a metal die 50 during cutting by a lathe;
FIG. 9 is a drawing showing a cross-section of a metal die; and
FIG. 10 is a perspective view of a new tool according to this invention.